Phytobilin biosynthesis: the Synechocystis sp PCC 6803 heme oxygenase-encoding ho1 gene complements a phytochrome-deficient Arabidopsis thaliana hy1 mutant

The phytobilin chromophores of phycobiliproteins and phytochromes are biosy nthesized from heme in a pathway that begins with the opening of the tetrap yrrole macrocycle of protoheme to form biliverdin IX alpha, in a reaction c atalyzed by heme oxygenase. An Arabidopsis thaliana hy1 mutant was previous ly shown to be deficient in phytochrome responses, and these responses were regained when the plants were administered biliverdin IX alpha. A heme oxy genase-encoding gene, ho1, was recently cloned from the cyanobacterium Syne chocystis sp. PCC 6803. When ho1 was expressed in Escherichia coli, the cel ls produced active ferredoxin-dependent soluble heme oxygenase. The open re ading frame of ho1 was fused in frame with a chloroplast transit peptide-en coding sequence from the oli gene of Antirrhinum majus. This construct was placed in a binary plasmid vector containing a kanamycin resistance marker and a cauliflower mosaic virus 35S promoter to control expression of the ch imeric oli-ho1 gene and used to transform A. thaliana hy1 plants. Two indep endent transformed lines were obtained that had the phenotype of the parent al Landsberg erecta line and expressed the chimeric gene, as indicated by d etection of its mRNA by reverse transcriptase-polymerase chain reaction. Th e results indicate that Synechocystis sp. PCC 6803 heme oxygenase encoded b y ho1 can substitute for the defective HY1 gene product and that the only r equired enzyme activity of the HY1 gene product is heme oxygenase.